This test was intermittently timing out, mostly on debug builds, because there
was a chance that from time to time the sidebar could have been loaded for the
first time while the background page of the test extension was still being loaded.
The test was sending an extension test message as soon as the sidebar was loaded
for the first time, and so if the background page didn't yet added its test message
listener, the test cases did never get started and the entire test was getting stuck.
The changes included in this patch prevents this intermittent failures by ensuring
that the test always waits for the background page and the sidebar to be both loaded
and ready before sending the test message that starts to run the test cases.
Differential Revision: https://phabricator.services.mozilla.com/D20657
6e435bd5c3